The X2 prototype superhelicopter had a successful first flight yesterday, according to its makers Sikorsky. The revolutionary (cough) aircraft hovered and manoeuvred using its twin main rotors without problems, but trials of its tail propulsor will take place on a future flight.
“Today’s achievement is the result of dedicated …

COMMENTS

Not all new technology

As far as I can see this is an extension of Sikorsky's own XH-59A "Advancing Blade Concept" (ABC) Technology Demonstrator programme from the late '70s. This managed to get up to 238 knots, so not far short of the 250 knot cruise they're aiming for.

Anyway, looks a better solution than most to the high-speed VTOL problem, with the advantage that the blade disk is smaller than a conventional helicopter so ideal for shipboard use.

Lewis, still here?

Hmm

And how, exactly, is this much of a development over the Lockheed AH-56 Cheyenne gunship, which was designed forty years ago? The basic principle of using an auxiliary lift source and a secondary propulsor to overcome the speed limitation of rotors was explored decades ago.

Granted, the Cheyenne used a small wing instead of a pair of contra-rotating rotors, and was thus technically a gyrodyne, and also had to have an anti-torque rotor as well as a pusher prop, but it still manage 212 knots.

Personally, I think that they should spend more time trying to get the jet-powered Rotor Wing to work. I always thought it was far cooler. It was about the only part of The Sixth Day that was worth watching.

Supersonic Tips

RE: Neil Hoskins

It's likely that the vibrations from passing in and out (accelerating past and decelerating back) of the sound barrier would create a large amount of undesirable vibration which would in turn damage/stress the rotors.. at a guess.

Good Article

Booms

Re: Supersonic tips

Supersonic tips are a lot less efficient than subsonic, because of the energy lost to the shockwaves ; and because supersonic lift is an entirely different beast than subsonic lift, requiring another (usually thinner) type of blade profile to be worth the extra effort. In addition the supersonic schockwaves can cause serious damage to the blade's material, generating creaks and cavities, leading to early failure. At higher, hypersonic, speeds there may even occur various undesirable chemical reactions when the air molecules become ionised and possibly split into their atomic constituents, damaging the surface some more. That is, provided the friction heat hasn't already melted or burned the blades.

@Simon Ball

The general problem with jet powered rotors (i.e where you have a little jet in each tip to spin it around) is that it's all kinds of noisy, whereas in a normal helicopter the free power turbine attenuates a lot of the jet noise.

I think if you youtube the Fairey Rotodyne you should get some sort of idea.

Re: Booms

Yes, they would and do.

You won't likely hear helicopter rotors going supersonic, however if you have been to an airshow you have probably had an example without realising it.

As Neil Hoskins mentions above, the North American AT-6 (Harvard/Texan) WWII trainer <http://www.airbum.com/pireps/PirepT-6.html> is very good at producing a lot of noise without going anywhere very fast. The distinctive and loud "Brrrrrrraaaaaaaapppp" sound is not produced by the engine, but rather by the propeller as the tips go supersonic. That's why the sound seems to come and go. As RPM reach a certain point the blades boom, below that speed they are much quieter (and more efficient).

Other airshow aircraft like the Stearman and other biplanes are good at this, as are some personal aircraft such as the Beechcraft Bonanza (when equipped with a 2 bladed propeller). That's not to suggest that they're flying around like this all day, but at maximum RPM they can make a lot of noise.

Um...

I hate to say it, but unless I'm very much mistaken most helicopters can vary the speed of the blades in flight, as well as the pitch - that's what the twist throttle on the collective is for is it not?

@SkippyBing

A rotor/wing doesn't have to use rotodyne-style tip jets. Because the rotor has to act as the primary lifting surface when stopped, as well as when in motion, it's much wider and thicker than a normal rotor. Consequently, it has space for a wider duct, allowing it to channel the engine exhaust directly, rather than needing a compressor (modern composites also allow much higher pressures and temperatures for a given duct weight).

It also means the the throat of the rotor nozzles can be wider, thus reducing the exit pressure, and thus the noise.

@Daniel Wilkie

Almost Lewis...

"The X2 has two main rotor assemblies stacked atop one another and rotating in opposite directions, a configuration long used by Russian helicopters. However, Sikorsky has added the ability to vary the rotors' speed in flight. Most rotorcraft spin their blades at a fixed rate."

Yes.. well done Lewis. It is also true however that while the Russian co-axes have conventional fully articulated rotor heads, the X2 uses Sikorsky's new semi-rigid 'advancing blade concept'. Thus making it completely different to the Russian aircraft.

The ABC is pretty much the whole point of this aircraft, so it would have been nice for it to get a mention.

@ Daniel Wilkie

I think you'll find that most choppers operate with a Constant Speed Unit to maintain stable rotor speed in flight.The throttle is necessary for throttling up in the first place from a start or to take off but in flight more throttle would only be needed if climbing or lifting, more energy will be used but without higher velocity of the rotor. A bit like driving a car up a hill at thirty miles an hour compared to driving it on the flat at the same speed, you need more gas to go up the hill at the same speed as you are doing more work. If anybody knows better please post.

@ Daniel Wilkie

Although technically you can alter the RPM of a helicopters rotors in flight (it's slightly harder in a multi-engine 'copter you generally need three hands) it's generally a bad thing. The whole set up is optimised to work with the blades spinning at a given RPM, you then get the engines to respond to changes in RPM by increasing or decreasing power as required via a governor, it's basically a step on from the technology in Chickenhawk. Bear in mind in most gas turbine powered choppers there's no direct connection between the engine and the rotors instead there's a free power turbine/windmill in the exhaust stream which then feeds into the gearbox.

Off the top of my head if you reduce the RPM to get the same lift you'll have to increase the pitch of the blades, which'll increase drag and place the blades nearer the stall. At higher speeds this'll be less of a problem as the pitch change will be less but it's all a bit complicated to be leaving to the pilot!

Technically, yes...

...but it's not a good idea. Helo blades need to be in a very specific speed range to generate the right amount of lift without risk of stalling. The twist-throttle is actually only present on helos without FADEC, where the pilot is responsible for keeping the rotors at the correct speed.

Because the stall happens on the retreating side first, it will cause the helo to roll violently if it is a single-rotor design. A twin-rotor design will stall one rotor on each side, which is why they *can* do it with the X2. Further, the slower rotor will have trouble giving up power to forward motion, which is why the X2 has the pusher.

Fascinating

shake

the problem here will be not the advancing blade, but the retreating one as the rotor is slowed down. The swash plate will dictate what angle of attack is on the retreating blade which will start to deflect in the airstream with less centrifical force holding it rigid. It talks about fiddling on the v-22. I see no (fiddling), this sounds like a single rotor axis argument from the v-22 nay sayers which thier ceo had who has moved to Bell espouses. He still says false things about the v-22, like half the lift, with half the time to targeted distances. He was the worst thing to happen to a solid program.

Lynx?

single engine isn't a benefit for this design either

and yes, the Osprey can run on a single engine. part of the complexity is the wacky transmission linkages required to transfer power in the event of a flame out.

No one's going to build a man-rated single engine military helicopter when a dual engine one gives them redundancy against battle damage. Plus, aren't all aircraft that are expected to operate over oceans (Navy, etc) *required* to be dual engine? Notice the Apache is still dual engined, So is the Navy variant of the AH-1. Neither is your upcoming Tiger ground support version, or the Russian Hind series.

The devil is in the details, and this is no Cheyenne. The ability to modify the rotor speed of two coaxial rotors, with serious rotating gyroscopic mass each, is a big deal. To some engineering ignorant hack (the type who thing "black boxes" are made of some super material to survive crashes) then it's no big deal.

But to us aviation nuts, and RC model (err...drone? RPV? UAV?) pilots, who've dug into this sort of thing since we cut our teeth on old modelling magazines, this prototype, and the CarterCopter are very exciting indeed.

So

So a modernised version of the auto gyro but with contra rotating blades and pusher engine and not pulling .

I seem to recall the flight handling characteristics were some what critical though and were very unforgiving in the hands of the fool hardy pilots who always pushed the envelope if I remember correctly .

Oh well in all probability the old O2B with superchargers on it's lycoming engines would be able to fly rings around this bird !

Rotor Clearance

The forward moving blade has more lift than the retreating blade and will bend upwards more. It is to be hoped that either the blades are stiff or there is plenty of clearance or it won't be whop whop whop like the Chinook but crunch crunch splat!

Did you know that the Chinook has vibration damping for the pilots seat. A big lump of concrete on a spring!

Another problem not yet mentioned with blade tips going supersonic is the stress you get with the end of the whirly bit (either heli's rotors or fixed-wing's propeller) moving at over 500mph (up to about 20,000ft IIRC) while the inner bit is moving only a few inches...

Heli rotors have an operating range of RPMs - it just tends to be rather narrow, with lift controlled by adjusting the pitch of all the blades equally and forward/directional 'speed' adjusted by adjusting the 'flapping angle' of the blades cyclically.

And don't the Ka-50/Ka-52 and Mil-28 have semi-rigid or rigid rotorheads? Yet another technology used first on a (Franco-)British development "acquired" by the Americans (just like the rotor tips now found on most American military helis are developed from the BERP tips pioneered by Westland a few years ago...)